Pulse width modulation is a technique whereby an analog signal can be sampled into a signal having two states (binary) by means of comparison of the analog signal with a reference signal. Pulse width modulation is used inter alia in class D amplifiers for audio applications. Class D amplifiers are known for their high power efficiency (low power losses) and relatively simple circuitry. The simple mechanical construction of these amplifiers has the result that they can be designed to be small and compact. Class D amplifiers are based on the principle that an incoming audio signal is compared with a periodic reference signal that comprises rising and falling edges in each cycle. A switched amplification is used herein such that an output signal of the connected amplifier unit is switched to a first voltage level when the voltage value of the audio signal is lower than the voltage value of the reference signal, whereas the output signal is switched to a second voltage level when the voltage of the audio signal is higher than the voltage of the reference signal.
A standard control circuit for pulse width modulation of a class D amplifier according to the prior art is schematically depicted in FIG. 1. FIG. 1 shows a circuit 1 based on a switched amplifier unit 9. An input signal 3 is applied to the circuit and is provided along with a reference signal 4 to the switched amplifier unit 9 via inputs 7 and 8. The switched amplifier unit 9 compares the voltage values between inputs 7 and 8. If the input signal has a voltage value that is higher than the voltage value of the reference signal, here offered via input 8, a high voltage signal VH 10 will be provided at the output of the amplifier unit. If the reference voltage Vref at the input 8 is higher than the voltage value Vc at input 7 of the switched amplifier circuit, however, a low voltage level VL 11 will be produced at the output of the switched amplifier 9. It is schematically indicated in FIG. 1 in the form of an adder unit 15 that the switched amplifier unit 9 adds a disturbance term Ve to the output signal. The output signal at an output 18 of the circuit is referenced Vo.
The circuit 1 is further characterized in that it comprises a feedback loop 20 which feeds the output voltage Vo back to the input 3 of the circuit, thus adding this output voltage Vo to the input signal Vi, which is schematically indicated as an adder unit 21. The feedback of the output signal Vo to the input of the circuit and the combination thereof with the input signal Vi is important for correcting various imperfections (collectively represented as a disturbance term Ve), in particular as regards the timing and phase in the power stage. An element 22 schematically indicates the transfer function H(s) of the circuit. This comprises both the transfer function of the forward path and that of the feedback path (the feedback filter). This element will be denoted the loop filter 22 hereinafter.
A problem of pulse width modulation circuits with feedback, such as the circuit 1 shown in FIG. 1, is the intermodulation between the input signal and the feedback signal. The output signal Vo consists of the desired average output voltage, disturbances, and a major switching residue, and accordingly comprises many high-frequency components, as will be discussed in more detail further below. The feedback signal consists of the complete pulse width modulated signal Vo. The feedback signal Vo is joined together with the input signal Vi and, after passing through the loop filter 22, is offered to the switched comparator amplifier 9 as a signal Vc. The loop filter 22 causes the signal Vo+Vi to be linearly deformed and changes the phase and/or amplitude of the spectral components of the signal Vo+Vi. The original rectilinear waveform, with regular rectilinear edges, will thus come to comprise (curved) line sections having slope angles that do not increase or decrease. The first derivative d(Vo+Vi)/dt is now no longer constant at the area of the comparator but will change in dependence on the modulation index (duty cycle). The switched amplifier 9 will deform in a non-linear manner, i.e. will add harmonics as a result of the curved edges of the signal Vc owing to the linear deformation of the loop filter 22.
The moment when in each cycle the output signal of the switched amplifier unit 9 switches from the first voltage level, for example VH, to the second voltage level, for example VL, is determined by the relative voltage levels of the reference voltage on the one hand and the voltage of the input signal on the other hand. The switch-over point between two voltage levels in the output signal Vo lies at the point of intersection of the edges of the reference signal Vref at input 8 and the signal Vc at input 7 of the switched amplifier. Vc has curved edges which in an ideal model ought to be straight and regular. The points of intersection between the edges of Vc and the edges of Vref accordingly do not lie in the locations where they should ideally be. The switch-over moment from the one voltage level to the other voltage level has thus become dependent on the deformation in the signal Vc, i.e. it is unknown. When a triangular signal is used as the reference signal (this is not the case in FIG. 1), the switch-over point from the first voltage level to the second voltage level, and back from the second voltage level to the first voltage level, will be unknown in both cases. The eventual output signal Vo accordingly is also deformed.
The non-linear behaviour of the circuit is aggravated by the fact that the spectrum of the output signal Vo varies continually with the modulation. The spectral properties of the output signal Vo can be explained as follows. Given an input signal Vi with a comparatively high voltage level, the output signal Vo will be at the second voltage level for a relatively short period of time in each cycle and will be at the first voltage level for the remaining part of each cycle. If on the other hand the voltage value of the input voltage Vi is comparatively low, the output voltage Vo will be at the first voltage level for a relatively short period of time in each cycle and will be at the second voltage level for a relatively long period of time in each cycle. The time during which the output signal is at one of the two voltage levels accordingly depends strongly on the voltage value of the input signal. This implies that the spectrum of the output signal, including the harmonics, will vary strongly with the modulation.
To reduce the problems of non-linearity, it has been proposed not to use a triangular reference signal, but instead, for example, a sawtooth signal. The use of a sawtooth signal in fact has the advantage that one of the edges of the reference signal falls (or rises) so steeply that the switch-over moment at the output of the switched amplifier 9 from the one voltage level (for example VH) to the other voltage level (for example VL) is substantially defined. This, however, does not solve the problem because it does not offer a solution to the spectrum of the output signal varying with the modulation.
International patent application WO/2006/030373 deals with the problem outlined above and also states that the use of a sawtooth reference signal has the result that at least one of the edges of each cycle of the periodic signal, i.e. the steep edge, does not carry any information. To improve the situation, the cited document proposes, for the input signal applied to the switched amplifier unit, an addition of an artificially created sawtooth frequency signal, to be regarded as a feedback signal, to this input signal upstream of the switched amplifier unit. The composite signal is then fed back to the start of the forward path, i.e. upstream of the filter, before being processed in the switched amplifier unit. In this switched amplifier unit the incoming composite and fed-back signal is compared with “0” in order to determine the zero passages. Switch-over points from the first voltage level to the second voltage level, and back from the second voltage level to the first voltage level, in each cycle are determined from the zero passages of the obtained composite and fed-back signal.
The effect of this method is that the deviation of the information stored in the pulse width modulated signal, i.e. the switching residue, is comparatively constant. The influence of the filter on the output signal is comparatively small as a result. A major disadvantage of this method is, however, that an assumption is made as to the signal that is to be fed back from the output of the pulse width modulation unit to the input signal. This feedback signal is a simulated one and is offered to the input signal without the switched amplifier 9 and the output filter (if present, not shown in FIG. 1) playing any part in the generation of this feedback signal. The final pulse width modulated signal is accordingly not used for the feedback in any way whatsoever, and such a method can only be implemented such that no new disturbances are introduced if the switched amplifier unit 9 and the pulse width modulation taking place therein are ideal. However, the reality is different: the operation of the switched amplifier unit 9 is not ideal, as is indicated schematically in FIG. 1, a component Ve being present in the output signal Vo at output 18 which has been added to the output signal of the switched amplifier unit 9 (adder unit 15). If this disturbance component Ve 14 is not known, the method described in WO/2006/030373 cannot lead to satisfactory results.
A further problem of the method described in WO/2006/030373 is that it is not easy to implement in that it uses a sawtooth generator immediately in front of the switched amplifier unit for providing the feedback signal. It is not simple to generate a pure sawtooth, which is why a digital sawtooth generator is used in WO/2006/030373.